This application claims the priority of German Patent Applications, Serial Nos. 100 03 874.3, filed Jan. 28, 2000, and 100 57 180.8, filed Nov. 17, 2000, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates to a method of joining steel tubes with aluminum ribs.
It is conventional to produce ribbed tubes for air-cooled plants or air-cooled condensers from steel tubes and steel ribs. Securement of the steel ribs to the steel tubes is realized through hot-galvanizing. Even though such ribbed tubes are very corrosion-resistant, they suffer the drawback that the ribs are made of relatively poor heat conducting steel.
It is further known to wind continuous aluminum ribs in helical manner onto round steel tubes. The aluminum ribs may hereby be wound with narrow foot-side legs under tension in flat engagement onto the surfaces of the steel tubes. Another possibility involves a grooving of the surfaces of the steel tubes and the placement of aluminum ribs in the grooves. A drawback of winding aluminum ribs onto steel tubes resides in the different thermal expansion coefficients between steel and aluminum. In practical terms, this means that steel tubes with aluminum ribs can only be used up to relatively low temperatures of about 130xc2x0 C. At higher temperatures, the contact between the aluminum ribs and the steel tubes gets lost as a consequence of the greater thermal expansion of aluminum. The capability of the ribbed tubes drops.
A further conventional method of making ribbed tubes involves the connection of aluminum-plated flat tubes with the assistance of aluminum-silicon solder with undulated or meandering or angularly folded webs of aluminum ribs in an annealing furnace. The bond between aluminum ribs and flat tubes through soldering with aluminum-silicon solder, which is a component of the aluminum ribs or the flat tubes, has the drawback that such soldering can be produced only via the detour of using aluminum-plated flat tubes or plated aluminum ribs. Apart from the comparably high expenditure in view of the need for diverse starting materials and during production, there is the added drawback that the flat tubes, closed in circumferential direction by at least one longitudinal welding seam, must not be plated with aluminum in the welding zone. Otherwise, a reliable welding operation cannot be assured. These regions of the flat tubes must be freed subsequently from welding by-products and then protected against corrosion.
A further drawback of ribbed tubes consisting of flat tubes with folded aluminum bands is the need to carry out the soldering operation of the aluminum-plated flat tubes with aluminum ribs at comparably high temperatures in the order of about 600xc2x0 C., i.e. near the softening temperature of aluminum. The solder, required hereby, is made of aluminum-silicone eutectic which melts slightly below the softening point of aluminum.
Also, with respect to this structural type, it is to be noted that as a consequence of the different thermal expansion coefficients between aluminum and steel, both soldered materials may substantially distort relative to one another, when cooled down to the ambient temperature (room temperature) after soldering at about 600xc2x0 C., so that a breakup of the soldered areas can easily be encountered, when the aluminum has not been correctly applied.
It would therefore be desirable and advantageous to provide an improved method of joining steel tubes with aluminum ribs as components of air-cooled plants and air-cooled condensers, which method obviates prior art shortcomings and can be carried out with slight labor costs and energy costs as well as cost-savings with respect to material consumption.
According to one aspect of the present invention, in a method of joining a steel tube with an aluminum rib, a layer of a zinc-aluminum alloy with an aluminum content of 0.5% to 20% is first applied onto the surface of the steel tube or the aluminum rib, and then a fluxing agent in the form of cesium-aluminum tetrafluoride is deposited between the steel tube and the aluminum rib at room temperature before or during mechanical contact of the aluminum rib with the steel tube, whereupon the steel tube provided with the aluminum rib is heated in a furnace to a soldering temperature between 370xc2x0 C. and 470xc2x0 C., and finally subjected to room temperature to cool down.
According to another aspect of the present invention, in a method of joining a steel tube with an aluminum rib, a layer of a zinc-aluminum alloy with an aluminum content of 0.5% to 20% is first applied onto the surface of the steel tube or the aluminum rib, and then a fluxing agent in the form of cesium-aluminum tetrafluoride is deposited upon the aluminum ribs at room temperature at least in the contact zone with the steel tube, whereupon the aluminum rib with the steel tube which has previously been heated to a soldering temperature between 370xc2x0 C. and 470xc2x0 C., are brought into mechanical contact, and finally subjected to room temperature to cool down.